The patent disclosure EP 1 232 988 A1 shows an elevator system with a position-determination system for determining a position of an elevator car. This position-determination system comprises a camera, which is arranged on the elevator car, and is used to generate images of hoistway material and/or of the surface structure of this hoistway material. To be regarded as hoistway material are guiderails and further elevator components that are arranged in the hoistway, as well as the hoistway walls that bound the hoistway. The hoistway material in its entirety forms a surface structure which essentially extends along the travel path of the elevator car. This surface structure constantly changes, so that each generated image of the surface structure is unique and can serve as an indicator for a position of the elevator car. In a learning travel, the camera generates reference images of the surface structure. An analysis unit that is connected with the camera identifies these reference images to a position in the hoistway and writes the reference images and the identified position values to a memory medium. In normal operation, based on a comparison of the images that are constantly generated by the camera with the reference images that have been saved, a position of the elevator car can be determined by the analysis unit.
For the robustness of the determination of the position, vibrations in the elevator car prove to be particularly critical. For example, the vibrations act negatively on the maintenance of a constant distance between the camera and the surface structure, since an object that is photographed by the camera appears, at a lesser distance, larger than at a greater distance. This variance in the distance from the surface structure presents the analysis unit, when determining the position, with special challenges since, for example, the size of the reference images does not match that of the constantly generated images and hence impedes a unique identification of the reference images. In addition, with a variable distance between the camera and the surface structure, it is not possible to deduce the true dimensions of the pattern based on the pattern that is identified from the images. For this, patent disclosure EP 1 232 988 A1 does not propose a solution.